1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of transmitting power information in a wireless communication system.
2. Related Art
To maximize performance and communication capability of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention in recent years. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. In this case, the weight can be expressed by a weight vector or a weight matrix, which is respectively referred to as a precoding vector or a precoding matrix.
The spatial multiplexing is classified into single-user spatial multiplexing and multi-user spatial multiplexing. The single-user spatial multiplexing is also referred to as single user MIMO (SU-MIMO). The multi-user spatial multiplexing is also referred to as spatial division multiple access (SDMA) or multi user MIMO (MU-MIMO). A capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel can be decomposed into independent channels. If the number of Tx antennas is Nt, and the number of Rx antennas is Nr, then the number of independent channels is Ni where Ni≦min{Nt, Nr}. Each independent channel can be referred to as a spatial layer. A rank represents the number of non-zero eigen-values of the MIMO channel and can be defined as the number of spatial streams that can be multiplexed.
The wireless communication system needs to estimate an uplink channel or a downlink channel for data transmission/reception, system synchronization acquisition, channel information feedback, etc. Channel estimation is defined as a process for restoring a Tx signal by compensating for signal distortion when fading occurs due to rapid environmental changes. In general, a reference signal known to both a transmitter and a receiver is required for the channel estimation.
In downlink transmission, a base station (BS) transmits a signal by determining Tx power suitable for data transmission performed by a user equipment (UE) located in a cell edge while minimizing inter-cell interference. The BS has to report information on the determined Tx power to the UE. Tx power for a reference signal may be set differently from Tx power for a data signal, and information on the Tx power for the reference signal may be indicated to the UE. The UE may receive the reference signal and the data by using Tx power information on the reference signal. Since each antenna experiences a different channel in a multiple antenna system, a reference signal arrangement structure is designed by considering each antenna.
Accordingly, there is a need for a method of effectively transmitting information on Tx power for a reference signal and data in a reference signal arrangement structure of various configurations.